Local IP based services may be accessed in the home, office, public hot spot or even outdoor environments. One of the important use cases for the local IP access and local connectivity involves the direct communication between devices in the close proximity (typically less than a few 10 s of meters, but sometimes up to a few hundred meters) of each other.
This direct mode or device-to-device (i.e. D2D) enables a number of potential gains over the traditional cellular technique, because D2D devices may be much closer to one another than cellular devices that have to communicate via cellular access point (e.g., eNB):                Capacity gain: First, radio resources (e.g. OFDM resource blocks) between the D2D and cellular layers may be reused (reuse gain). Second, a D2D link uses a single hop between the transmitter and receiver points as opposed to the 2-hop link via a cellular AP (hop gain).        Peak rate gain: due to the proximity and potentially favorable propagation conditions high peak rates could be achieved (proximity gain);        Latency gain: When the UEs communicate over a direct link, eNB forwarding is short cut and the end-to-end latency can decrease.        
D2D communication or operation may also be called direct link or direct mode communication or operation or peer-to-peer communication or operation or sidelink communication or operation or ProSe (proximity services) communication or operation.
A wireless connection for D2D operation or communication may be called sidelink, it may refer to a direct communication link and/or radio link between two wireless devices or UEs (user equipment). D2D communication and/or such a link may generally be provided via unicast, multicast and/or broadcast.
Recently, device-to-device (D2D) communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Typically, it is suggested that such device-to-device communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the device-to-device services and cellular services is more flexible and provides higher spectrum efficiency.
D2D applications include direct discovery and direct communication. In both cases, the transmitter sends D2D signals that should be directly received at least by the intended receivers.
D2D should also be able to operate in multi-carrier scenarios where cellular and/or D2D is configured to operate on multiple carriers. Such carriers do not necessarily belong to a single operator and are not necessarily coordinated and synchronized.